This invention relates to a speed control system for an a.c. electric motor which is connected in series with a controllable switch between first and second a.c. supply conductors, the system comprises a data processing system comprising a program store and a digital data processor for performing a program of processing operations on digital data in accordance with program data contained in said program store, the processing system has an output coupled to a control input of said switch, a detector for zero potential between said supply conductors, which detector has an input coupled to said supply conductors and an output coupled to an input of said data processing system for generating an output signal when zero potential occurs between said supply conductors, a tachogenerator having an input coupled to said motor and an output coupled to an interrupt signal input of said data processing system, and a counter having an input coupled to an output of a clock signal source and an output coupled to an interrupt signal input of said data processing system for generating an output signal when count therein, reaches a predetermined value. The program comprises a cyclic main program and first and second interrupt routines which are arranged to be performed in response to the generation of an output signal by said tachogenerator and in response to the count in said counter reaching said predetermined value respectively.
Such systems have to perform three main functions, namely
(i) determine the period of the tachogenerator output signals and hence the actual speed of the motor,
(ii) calculate the delay time or firing period from each zero-crossing in the a.c. supply after which the switch is required to conduct (often referred to as the "firing angle") and time this delay, and
(iii) cause the switch to conduct at the end of the time delay.
Due to the overlapping timing requirements of the tachogenerator signal period and of the firing angle, and the arbitrary nature of the relationship between the instants determining the two timing periods, it is known to use a first counter/timer for measuring the tachogenerator period and a second counter/timer for generating the firing angle time delay. U.S. Pat. No. 4,584,504 discloses a system of the kind defined in the first paragraph which enables one of these two timer/counters to be dispensed with. In this known system the main program is arranged to be synchronised with the output signals generated by the detector, each cycle occupying a time which is less than that which elapses between two successive such output signals and a wait occurring until each such output signal occurs. This is wasteful of potential processing time in the main program and it is an object of the present invention to remove the necessity for such waiting periods. Moreover, the processing in the known system is quite complicated and it is a further object of the present invention to enable this processing to be simplified.